Introduction To Perl
Susan Lukose
Introduction to Perl
• Practical Extraction and Report Language
• Easy to learn and use
Perl Variables
•
Scalar : An individual value (number or string). A scalar variable is preceded by $.
$answer=42;
$pi=3.14;
$pet=“dog”;
$product=$answer*$pi;
$mystring=“ I have pet ” .$pet;
•
Array : An ordered list of scalars, accessed by scalar’s position in the list. A array variable is preceded by @.
@home=(“couch”, “chair”, “table”);
$home[0]=“couch”;
$home[1]=“chair”;
$home[2]=“table”;
•
Hash : An unordered list of scalars, accessed by some string value that is associated with each scalar. A hash
variable is preceded by %.
%longday= (
“sun”=>”Sunday”,
“mon”=>”Monday”,
“tue”=>”Tuesday”,
);
$longday{“mon”}; will have the value Monday
•
Subroutine : A callable chunck of perl code. A subroutine variable is preceded by &.
sub mysub {
command;
}
&mysub; To call the subroutine
Operators
•
Assignment Operator:
$var = "string"; Puts the string into $var
$var = 5; Puts a number into $var
$var = $var. "string"; Appends string to $var
•
•
Comparison Operator:
Comparison
Numeric
String
Return Value
Equal
==
eq
True if $a is equal to $b
Not equal
!=
ne
True if $a is not equal to $b
Less than
<
lt
True if $a is less than $b
Greater than
>
gt
True if $a is greater than $b
Comparison
<=>
cmp
0 if equal, 1 if $ a is greater, -1 if $b is
greater
Logical Operator: AND,OR and NOT are regularly used in if() statements
if($first && $second){....;}
if($first || $second){....;}
if($first && ! $second{....;} means that $first must be true but $second must be false.
•
File Test Operator:
-f $file
-d $file
-r $file
-x $file
-w $file
-e $file
It's a plain file
It's a directory
Readable file
Executable file
Writable file
File exists
Branching and Looping
•
if
if(condition){
command;
}elsif(condition){
command;
}else{
command;
}
•
while
while(condition){
command;
}
•
for
for($variable=0;$variable<condition;$variable++)
{
command;
}
•
foreach
for $info (@data){
print $info,"\n";
}
Regular Expression
A regular expression is an abstract formulation of a string making it possible to do complex string comparisons, selection,
replacements and parsing.
•
=~
: This operator appears between the string var you are comparing.
$string =~ m/Bill Clinton/;
#return true if var $string contains the name of the president
$string =~ s/Bill Clinton/Al Gore/; #replace the president with the vice president
•
!~
: Just like =~, except negated.
•
/
: This is the usual delimiter for the text part of a regular expression.
$string =~ m/Bill Clinton/;
#return true if var $string contains the name of the president
$string =~ s/Bill Clinton/Al Gore/; #replace the president with the vice president
•
m
: The match operator.
$string =~ m/Bill Clinton/;
$string =~ /Bill Clinton/;
•
s
#return true if var $string contains the name of the president
#same result as previous statement
: The substitution operator.
$string =~ s/Bill Clinton/Al Gore/; #replace the president with the vice president
•
^
: This is the "beginning of line" symbol.
$string =~ m/^Bill Clinton/;
•
$
: This is the "end of line" symbol.
$string =~ m/Bill Clinton$/;
•
i
#true only when "Bill Clinton" is the first text in the string
#true only when "Bill Clinton" is the last text in the string
: This is the "case insensitivity" operator when used immediately after the closing delimiter.
$string =~ m/Bill Clinton/i;
#true when $string contains "Bill Clinton" or BilL ClInToN"
Regular Expression
•
Wildcards
. Match any character
\w Match "word" character (alphanumeric plus "_")
\W Match non-word character
\s Match whitespace character
\S Match non-whitespace character
\d Match digit character
\D Match non-digit character
\t Match tab
\n Match newline
\r Match return
•
Repetitions
* Match 0 or more times
+ Match 1 or more times
? Match 1 or 0 times
{n} Match exactly n times
{n,} Match at least n times
{n,m} Match at least n but not more than m times
$string =~ m/\s*rem/i; #true if the first printable text is rem or REM
Perl Build-in Functions
Get all upper case with:
Get only first letter uppercase:
Get all lowercase:
Get only first letter lowercase:
Get the length of a string:
Extract 5-th to 10-th characters from a string:
Remove line ending:
Remove last character:
Show position of substring in string:
Show position of last substring in string:
Print a string
Splits up a string and places it into an array
$name = uc($name);
$name = ucfirst($name);
$name = lc($name);
$name = lcfirst($name);
$size = length($string);
$part = substr($whole,4,5);
chomp($var);
chop($var);
$pos = index($string,$substring);
$pos = rindex($string,$substring);
print (“My name is ”.$name);
$info = "Caine:Michael:Actor:14, Leafy Drive";
@personal = split(/:/, $info);
which has the same overall effect as
@personal = ("Caine", "Michael", "Actor", "14, Leafy Drive");
Examples
•
Print Hello World
#!/usr/bin/perl
print ("Hello World\n");
•
variables
#!/usr/bin/perl
$a=10;
$b=$a*100;
print ("Product of ".$a." times 100 is ".$b."\n");
•
Comparisons
#!/usr/bin/perl
$str1="product";
if($str1 eq "sum")
{
$a=10;
$b=$a+100;
print ("Sum of ".$a." and 100 is ".$b."\n");
}
elsif($str1 eq "product")
{
$a=10;
$b=$a*100;
print ("Product of ".$a." times 100 is ".$b."\n");
}
Examples
•
Print file names of the current directory
#!/usr/bin/perl
use Cwd;
$dirtoget=getcwd;
opendir(IMD, $dirtoget) || die("Cannot open directory");
@thefiles= readdir(IMD);
closedir(IMD);
foreach $f (@thefiles)
{
print ($f."\n");
}
•
Reading a file and looking for a match in the content
#!/usr/bin/perl
$data_file="student.txt";
open(DAT, $data_file) || die("Could not open file!");
@raw_data=<DAT>;
close(DAT);
foreach $stu (@raw_data)
{
if($stu =~/unix/i)
{
print($stu."\n");
}
}
Examples
•
Print perlfile names of the current directory
#!/usr/bin/perl
use Cwd;
$dirtoget=getcwd;
opendir(IMD, $dirtoget) || die("Cannot open directory");
@thefiles= readdir(IMD);
closedir(IMD);
foreach $f (@thefiles)
{
if($f=~/(.+).(pl$)/i && $f!~/^\./i)
{
print ($1."\n");
}
}